This invention relates to power controllers. More specifically, this invention relates to power controllers for amplitude modulation in the radio frequency.
An RF power amplifier amplifies an RF input signal and drives a load, such as an antenna. The RF power amplifier determines the strength of an RF signal delivered to the load. For an antenna, the RF power amplifier determines the strength of an RF signal radiated by the antenna. In many applications such as cellular telephones and wireless data modems, the amount of RF energy delivered to the load must be tightly controlled within a specified range of power levels. Therefore, an RF power controller is often used to regulate the output of an RF power amplifier. The RF power controller determines the degree of amplification applied to the RF signal and thus the power level of a signal transmitted to the load.
An RF power controller may comprise an open loop or closed loop design. In an open loop system, the RF power controller compares a power control input signal, which indicates the desired output power level, with a fixed reference signal. The difference between the signals controls the amplification factor of the RF power amplifier.
In a closed loop system, the actual output of the RF power amplifier is compared with the power control input signal. The RF power controller adjusts the amplification factor of the RF power amplifier to minimize the difference between the measured power level and the desired power level. Closed loop control systems provide more accurate regulation of the output power level than open loop systems.
RF power amplifiers used for mobile communication output a power signal with a constant amplitude to a load based upon a phase modulated RF input signal. A new standard, referred to as EDGE, that is used by GSM phones increases data output rates by using amplitude modulation as well as phase modulation. An RF power controller cannot use a closed power control loop to control the output power of the RF power amplifier with the EDGE standard during amplitude modulation (AM). In EDGE systems, the output power signal of the RF power amplifier has a non-constant amplitude. A closed power control loop tends to cancel out the amplitude modulation in the power output signal in response to variations in the amplitude of the feedback signal, which is undesirable.
It would, however, be desirable to provide an RF power controller that accurately controls an amplitude modulated output power signal of an RF power amplifier using the EDGE standard or other standards such as the TDMA standard.
It is therefore an object of the present invention to provide an RF power controller that accurately controls an amplitude modulated output power signal of an RF power amplifier using the EDGE standard or other standards such as the TDMA standard that have non-constant amplitude power modulation.
It is also an object of the present invention to provide an RF power controller that regulates in open loop when the power output signal of the RF power amplifier is amplitude modulated.
It is also an object of the present invention to provide a system for maintaining a constant RF power controller output voltage during open loop.
It is also an object of the present invention to minimize voltage transients in the output voltage of the RF power controller when going from open loop to closed loop.
In accordance with this and other objects of the present invention, RF power controllers that provide an accurate power output signal from an RF power amplifier with amplitude modulation is provided. The present invention also includes methods for accurately controlling the RF power output signal of an RF power amplifier with amplitude modulation. RF power controllers of the present invention control an amplitude modulated power output signal from an RF power amplifier using the EDGE standard or other standards such as the TDMA standard that have non-constant amplitude power modulation. RF power controllers of the present invention close the power control loop during the-ramping up and subsequent ramping down of the RF power output signal in an enable cycle.
The RF power output signal is amplitude modulated during a period of time in between the time it is ramping up and ramping down. During the period of time that the RF power output signal is amplitude modulated, the power control loop is open, and the output voltage of the power control amplifier is stored on a capacitor followed by an amplifier. The amplifier output supplies the output voltage of the RF power controller to the RF power amplifier during the amplitude modulation period of time. The average value of the RF output power signal remains constant, because the power control output voltage supplied by the amplifier is constant.
The present invention may also include additional circuitry for further ensuring that the output voltage of the RF power controller remains substantially constant during the open loop period of time. The differential voltage at each input of the amplifier may be maintained on two capacitors during open loop. The present invention minimizes voltage transients in the output voltage of RF power controller as it transitions from open loop to closed loop by controlling the output voltage of the power control amplifier during open loop.